1. Field of the Invention
The present invention relates to a method for continuously forming a structural member using a composite material.
2. Description of the Related Art
One typical method for forming a structural member using a composite material mainly composed of carbon fiber is the forming method using an autoclave device.
FIG. 16 illustrates the outline of the manufacturing method using an autoclave device 600, wherein the composite material used in the autoclave forming method is a material called a carbon fiber/epoxy prepreg, in which a carbon fiber fabric is impregnated with epoxy resin.
In this method, nitrogen gas is filled in a can 600 of the autoclave device, and the pressure and temperature of nitrogen gas is raised to press and heat a material 650 sealed in a vacuum bag 610 so as to form the material. According to this method, it becomes possible to form a high quality structural member 650 having surfaces of second/third-order curvature, or other complex shapes.
However, the prior art method has inferior production efficiency and high costs, since the method requires a large-scale autoclave device 600 and a large mold (molding jig) 620, requires molding processes involving manpower called lay-up and set-up, and produces only a limited number of products in a single molding process.
[Influence of Thermal Expansion]
In addition, the method has a drawback in that the influence of thermal expansion coefficient that differs according to the material causes undulation (fluctuation) of the carbon fibers F1 that constitute the fabricated structural member, by which both the strength and elastic modulus of the member are deteriorated, and as a result, the flexural rigidity in the low stress area is deteriorated.
The thermal expansion coefficient of the carbon fiber F1 itself as main material is zero or minus, but on the other hand, the thermal expansion coefficient of the epoxy resin P1 solidifying the fiber is as high as 65 PPM.
In the autoclave method, aluminum alloy having superior thermal conductance is used as the material of the mold (jig) 620 so as to improve the thermal conductance from the nitrogen gas to the prepreg material 650.
Since the thermal expansion coefficient of aluminum alloy is as high as 23 PPM, the mold 620 heated in the autoclave can is expanded greatly due to thermal expansion.
When the carbon fiber F1 is heated for 160° C. from the room temperature of 20° C. to the heating temperature of 180° C., it will not expand, but the resin P1 contained in the prepreg is greatly expanded via thermal expansion when the curing is completed, though it is somewhat constrained by the fiber F1.
Similarly, when the temperature is raised by 160° C., the thermal expansion of the mold 620 reaches 0.37% since it is not constrained by the carbon fiber F1, and the thermal expansion of the mold 620 having a length of 2 meters is as large as 7.4 mm.
When heated, the carbon fiber F1 is stretched via the resin P1 and the mold 620, and straightened.
When the heating/pressing step is terminated and a cooling step is started, the mold 620 and the cured epoxy resin P1 start to shrink.
At this time, since the carbon fiber F1 is not subjected to thermal contraction, and since it has high elastic modulus and thus is not subjected to stress contraction, the fiber is slightly buckled in a wavelike form (fluctuated state) when the step is terminated, as shown in the conceptual diagram of FIG. 17.
When tensile load is applied to the structural member fabricated as above, as shown in FIG. 18, the members shows a low elastic modulus until the buckling of the fiber is eliminated and straightened.
That is, in the initial low stress area, the stress/strain relationship is not proportional but is represented by a curve as shown in portion A of FIG. 18, and only after the fibers are straightened that the elastic modulus of the material itself will be seen, as shown in portion B of FIG. 18.
As described, in the low stress area, a large deformation (strain) is created and the elastic modulus is small.
On the other hand, if compressive load is applied, the buckling of the fiber is increased, so that the stress-strain diagram shows a low elastic modulus as illustrated in portion C of FIG. 18, which finally results in buckling.
When a group of fibers solidified by resin receives compressive load, the compressive elastic modulus and the buckling strength varies according to the initial straightness of the fibers, as shown in FIG. 19.
Line LA represents a case in which the fibers are straight, line LB represents a case in which the fibers are slightly undulated, and line LC represents a case in which the fibers are greatly undulated.
Even after the fibers are buckled, the surrounding resin supports the fiber, so the member exerts some level of stress.
The above-mentioned phenomenon is called “micro-buckling” in the Society for Composite Materials, and studies regarding the phenomenon are conducted.
One means for solving the problem of fluctuation is the use of a mold formed of a special metal material having zero thermal expansion coefficient called an inver instead of the mold made of aluminum alloy, but there are drawbacks in that the material is expensive, the processing thereof is difficult, and the thermal conductivity is as small as stainless steel, which elongates the time required for the heating step.
The above is a description of the forming method using an autoclave.
Another possible forming method is the hot press forming method.
The method is advantageous in that it has high productivity, and is suitable for manufacturing panel products, but due to limitations regarding the press mold and the pressurizing direction, the method is not suitable for forming structural members having a complex cross-sectional shape, or for forming long structural members.
When manufacturing a large-sized product, since it is difficult to ensure the surface accuracy of the press mold, it is difficult to manufacture an accurate structural member.
When steel having low thermal expansion coefficient is used to form the press mold of the hot press, thermal expansion of the press mold will not affect the product greatly, however, thermal contraction of resin will inevitably affect the product.
In addition to the above-mentioned forming methods, there is known another method so-called a pultrusion method for continuously forming a long composite material.
This method involves passing long fibers in a mold, pulling and forming the same, and simultaneously curing the resin impregnated in the fibers in a short time. The method not only has advantageous productivity, but since tension is constantly applied during the forming process, the undulation of fibers is minimized, so that the material characteristic is improved. However, the method cannot be applied to form a structural member having a complex cross-sectional shape or a structural member having high quality.
“ADP Forming Method”
An ADP forming method is a forming method having developed the pultrusion forming method, and the present applicant has acquired a patent related to the continuous forming method thereof in Japanese Patent No. 1886522 (patent document 1).
The forming method of patent document 1 (ADP forming method) utilizes a prepreg material in which a group of fibers are impregnated with resin and semi-cured in advance.
As shown in FIG. 20, the method is a continuous forming method comprising feeding a prepreg material 702 wound around a roll 701, overlapping necessary number of prepreg materials 702, passing the same through an injection mold 700, intermittently pressing and heating while pulling the member so that the impregnated resin is cured, and moving the member for a short distance during removal of pressure.
A device 700 for heating and pressing the prepreg material 702 has the same structure as a common small-sized thermoforming press, for intermittently heating and pressing the material.
The device at the center of FIG. 20-1 is a post-cure furnace 720 for completely curing the resin, wherein the material is moved from left to right while the resin is completely cured.
The device at the right end side of FIG. 20-1 is a device 730 for intermittently moving the material in correspondence with the pressure removal cycle.
A pressurizing cylinder 731 for applying friction is repeatedly moved via a sender cylinder 732, by which the material is moved.
FIG. 20-2 illustrates a structure of a mold for forming a T-shaped structural member, wherein an injection mold 750 of the heating and pressing device 700 is divided into three upper and lower parts 751, 752 and 753 having a cross-sectional shape corresponding to the structural member to be formed, each having a heating device 740 built therein.
The upper parts 751 and 752 of the mold are moved up and down via pressure cylinders 761 and 762, by which the material is intermittently pressed.
The pressing pressure is approximately 3 Kg/cm2, and the pressing cycle repeats pressing for 30 seconds and removing pressure for approximately 2 seconds.
During the pressure removal step, the material is moved for approximately 30 mm, so that the traveling speed thereof is approximately 3.4 m per hour.
The heating temperature is determined by the thermal curing property of the impregnated resin, which is in the range of 120 to 180° C., and the structural member having passed through the curing furnace and completely cured is then cut into predetermined lengths with a saw.
The above is a description of the ADP forming method.
The ADP forming method is advantageous compared to the pultrusion method since it can form a structural member with high quality and complex material configuration, and since tension is applied to the fiber though slightly during the forming process, the “fluctuation” of fibers is reduced and the material characteristic as a member is improved. However, there is a limitation to the cross-sectional shape of the member to be formed due to limitations regarding the shapes of molds for heating and pressing the material and the limitations regarding pressing direction.
Structural members having intense curvature, structural members having a hollow structure and structural members having torsion are demanded as structural members of aircrafts, but such structural members are difficult to form using the ADP forming method.
Therefore, the applicant of the present invention has conducted further researches and acquired patents related to the continuous forming method of a composite material member having a certain curvature in Japanese Patent No. 3402481, No. 1886560, No. 3012847 and No. 3742082. However, the cross-sectional shapes and curvatures of the members to be formed are still limited.
The structural members using advanced composite materials are light-weight and have high strength, but the method for forming the same are not yet developed, so the advanced properties of the materials are not fully utilized.